A number of lifts have been developed in order to aid disabled persons, in particular, wheelchair passengers, in boarding and exiting vehicles such as buses, trains, vans, and the like. One type of lift, commonly known as a "step lift," is disclosed in U.S. Pat. No. 4,466,771 to Thorley et at. ("the '771 patent"). The step lift is designed to be installed in the stairwell of a transit vehicle, and includes hinged panels that are movable between a step position and a platform position. In the step position, the hinged panels form steps for use by passengers to board and exit the vehicle. In the platform position, the hinged panels form a horizontal platform for use to raise and lower a wheelchair passenger between a vehicle's floor-level position and a ground-level position. The hinged panels are attached to a carriage assembly that may be driven upward or downward to raise or lower the platform.
A second type of lift, commonly known as a "platform lift," is designed to be used in a vehicle where steps are not needed. In a platform lift, a solid platform rests in a vertical or horizontal position when not in use, and may be hidden underneath the vehicle. The platform is lowered to the ground or curb, and raised to its resting position, in order to transport the passenger between the ground and the vehicle.
Typically, passengers board and exit a bus from a curb along the edge of a street. A lift generally is configured such that the carriage assembly has a lowermost point of travel that enables lowering of the platform to a street level position in order to accommodate wheelchair passengers in those instances in which boarding or exiting of the vehicle occurs from the street level rather than from a curbside. Thus, in cases where there is a curb, the height of the curb is above the lowermost point of travel of the cartage assembly. It is important to stop downward movement of the cartage assembly, i.e., lowering of the platform, at the time that the platform contacts a curb or other object. If lowering of the platform is not stopped at that time, the lift may be damaged, a passenger on the lift may be frightened and/or injured, and/or the lift may jack the vehicle.
A number of different arrangements for stopping lowering of a lift platform when the platform contacts a curb or other object have been developed. For example, the '771 patent discloses a wheelchair lift in which a sensitive edge is provided on the bottom of the platform. When the sensitive edge contacts an object, a switch is actuated to stop the motor, and hence lowering of the platform. Since it is located on the underside of the platform, the sensitive edge is susceptible to damage from ground abrasion and also to deterioration from rust, dirt, ice, or the like.
U.S. Pat. No. 5,299,904 to Simon et at. ("the '904 patent") discloses a lift with a contact sensor that is protected from the environment. The lift disclosed in the '904 patent includes two rails attached to a vehicle, and two slides that ride upon the rails. A carriage and a platform are attached to the slides. The carriage and platform normally rest in a position above the level of a curb or the ground. As the platform is lowered to the curb, the attached slides extend away from the vehicle, riding upon the rails.
A contact sensor is situated on the bottom surface of the top part of the slide, so that it can detect whether the top of the rail is contacting the slide at the point of the sensor. As the platform is lowered to the curb, prior to contact with the curb, the weight of the platform is supported entirely by the rail. After the platform contacts the curb and continues to be lowered, a portion of the weight is transferred from the rail to the curb. As the platform continues to be lowered, the inner portion of the slide becomes lower than the outer portion of the slide. When this occurs, there is no longer contact between the rail and the slide at the point of the contact sensor. The sensor, detecting the change in position, sends a signal to a lift controller, which terminates the lowering of the platform.
One weakness of the '904 invention is that there is little tolerance between the slide and the contact sensor. Thus, as the vehicle on which the lift is mounted rocks back and forth once the lift platform is lowered to the curb, the rocking tends to repeatedly trigger the contact sensor. As the contact sensor is repeatedly triggered, it causes the outer safety barrier to attempt to move between an extended position and a safety barrier position. Thus, the contact sensor does not always provide a correct indication of whether or not the platform is properly lowered and rested upon the curb.
As can be seen from the discussion above, there exists a need for an improved apparatus to detect when the platform contacts the curb. The present invention is directed toward addressing this need.